The quality requirements of papermaking have grown at the same time as the tech- nologies that use paper have developed. In order to work reliably and continuously, the new and faster copying and printing machines require very smooth paper grade.

For example, variations in the thickness of paper both in the longitudinal and the cross machine direction can cause a break in the production of the printing machine.

In calendering in paper machines and paper finishing machines, rolls are used that form a nip with a backing roll. In multi roll calenders, there are several successive nips formed by pairs of rolls, through which the web is directed. To manage the process, it is important that the nip pressure, especially the distribution of the linear pressure (the profile) in the cross machine direction of the web, i. e., the axial direc- tion of the rolls, can be measured and, as necessary, adjusted.

The pressure in the calender nip can be measured in various ways. Publication US 5 383 371 presents a method for measuring the pressure in the nip both in the longitu- dinal and the cross machine direction of the web. The method uses piezoelectric detectors that can either be attached to the body of the roll or between the layers of the roll coating. The detectors are preferably manufactured of a polyvinylidenedi- fluoride film (PVDF), which is a piezoelectric film. The detectors can also be at- tached in two layers between the layers of the roll coating.

Publication US 5 562 027 discloses various detectors, such as piezoelectric and pie- zoresistive sensors, optical fibres, and stress-strain sensors. The piezoelectric and piezoresistive sensors are preferably made of thin film and placed on the roll so that they measure the radial pressure. When using stress-strain sensors, information or the radial pressure is indirectly obtained. On the contrary, the sensors that use opti- cal fibres are capable of measuring the pressure in any direction. The above-

described sensors can be located on the surface of the roll or between the layers of the roll.

The piezoelectric detector presented in publication US 5 383 371 comprises the PVDF film mentioned above, a metal pattern, such as an aluminium pattern, being arranged on both sides of the film. A protective film is arranged on top of the metal pattern. The metal-coated areas form detectors that measure the compressive force.

The metal patterns are connected by tape conductors, which are also made of metal- coated tapes. The measurement signals are directed to a monitor that shows the compressive force distribution. The measurement signals can also be directed to the control system of the calender, for example.

In publication US 5 383 371, signals can be transmitted by using, for example, a radio transmitter, an optical transmitter or some other technique known per se, which is used to cordlessly transmit the signal to a receiver, i. e., a signal processing unit. Furthermore, publication US 5 592 875 discloses a wired signal transmission system, whereby a slip ring or a rotary transformer is used, for example, when the whole roll rotates and the signal-processing unit is stationary. On the other hand, rolls of the type described above can also be used, comprising a stationary centre and a rotary envelope, whereby it is easy to transmit signals along conductors to the processing unit. Publication FI 92 771 also presents the transmission of measure- ment signals from a rotary roll to stationary receivers.

In order to know and control the papermaking process, it is important to know the pressure exerted on the paper web in the roll nip of the paper machine in the longs- ; tudinal and the cross machine direction. The intention can be to keep the pressure or the pressure distribution stable or that this distribution be adjustable as desired, for example, to regulate the thickness of the web in the cross machine direction of the web. By using the information provided by the nip pressure, a correct method can be defined to regulate the thickness profile of the paper web. Various possibilities for the adjustment are known. The most general method to correct the observed roughness of the web is to increase or decrease the compressive force exerted on the paper web in the nip, preferably by selectively changing the pressure in the cross machine direction of the web. Other methods used include regulating the thickness profile in the head box (adjustment of the slice) and heating or wetting the web.

Publication US 4 791 863 discloses a system for adjusting the pressure profile of the nip in the cross machine direction of the web by using zone adjusting rollers. The system comprises a zone adjusting roller and its counter member, such as a backing

roll, jointly forming a nip, through which the web to be processed travels. The zone- adjusting roller is a variable crown roll that has a stationary part, i. e., a massive cen- tral shaft, and a cylindrical roller jacket that rotates around the shaft. Slide shoes, which are grouped into pressure charge zones, work against the smooth inner sur- face of the roller jacket, each zone being loaded by pressure cylinders.

Publication US 4 903 517 presents a calender, wherein at least one nip is formed from a roll, a shaping cylindrical envelope being around the stiff bearing centre of the roll, and from its counter member. Furthermore, the stiff centre can be station- ary, whereby the envelope rotates around it. The shaping envelope is divided into parts that can be adjusted independently without the adjustment of one part affect- ing the other parts. The adjusting units are arranged in at least one row in the direct tion of the roll shaft.

Furthermore, the pressure profile of the nip can be regulated by changing the tem- perature of the roll either inside or outside the roll, whereby the thermal expansion in the roll provides a basis for the adjustment.

Adjustment of the nip pressure and, consequently, of the web's thickness profile is based on the measuring data, which are generally collected by a thickness sensor that is arranged after the nip, attached to a measuring bar, and moves transversely to the web. In that case, however, the measuring bar is far from the regulating ar- rangement and, in addition, the detector moves transversely, whereby there is a great delay in control. Furthermore, information is obtained on the final thickness of the paper only. On the other hand, various factors affect the actual pressure in the nip, including speed, the properties of the incoming paper, and the temperature, on which there is no accurate information. Consequently, there is no accurate estima- tion about the operation of the nip, especially the pressure in it, and any change in the thickness profile caused by the pressure.

The object of the invention is to provide a method for controlling the calendering of the web and a method and a system for quick and accurate regulation of the web's thickness profile.

These objects have now been accomplished as described in the appended claims.

The on-line method according to the invention for regulating the thickness of the web in the set of rolls of a paper or board machine is characterized in that

-the web thickness is measured in the machine before the nip between a pair of rolls, -the compression pressure exerted on the web is measured in the nip, -the web thickness is remeasured after the nip, and -on the basis of the measurement, the desired control actions are carried out to maintain or change the thickness of the web.

Thus, the method according to the invention collects measurement data about the properties of the paper web in the cross machine direction of the web immediately before the nip, in the actual nip, and immediately after the nip, whereby the thick- ness of the paper web is measured immediately before and after the nip and, in the nip, the compression pressure exerted on the paper web is measured. This informa- fion is combined in the data processing and control unit, whereby the relation be- tween the pressure in the nip and any change in the thickness caused by the pressure are detected. On the basis of this information, the loading of the roll can further be corrected either in accordance with a relative target of change or directly with the <BR> <BR> final target of change to selectively regulate the thickness of the web.

The compression pressure exerted on the web is preferably regulated in the nip by using a pressure medium acting on the dimensions of the roll or the temperature of the roll surface. The temperature of the roll surface provides thermal expansion or contraction of the roll, changing the dimensions of the roll. The surface temperature of the roll can be affected, for example, by changing the temperature of the roll jacket or by heating the roll surface externally by infrared rays, or inductively.

Furthermore, the thickness profile of the web can be regulated by selectively chang- ing the roll profile in the longitudinal direction of the roll in any of the above- described ways. In addition, the profile of the paper web can be affected by a sec- ondary control action, such as wetting the web.

The method according to the invention can be used, for example, in multi roll cal- <BR> <BR> <BR> enders or the press section of the paper or board machine. When the method is used ; in the multi roll calender, the compression pressure and the changes in the web thickness can be controlled simultaneously in one or more nips. This is preferable, when the control actions following the measurements are focused on various nips of the calender as selected. The multi roll calender can be, for example, like the one

described in publication FI 96 334 or WO 98/50 628. Furthermore, the rolls of the multi roll calender are preferably variable crown rolls.

When the method described above is used in the multi roll calender, the measure- ments are preferably carried out in at least one nip, preferably the first nip in par- ticular, whereby the web thickness is preferably regulated by changing the compres- sion prexssure exerted on the web in at least one nip, which is not the measurig nip; the web thickness is more preferably regulated in the seond nip of the multi roll calender. Furthermore, the method can be used simultaneously in several nips of the multi roll calender.

Another object of the invention is a system for applying the method described above in the set of rolls of a paper or board machine, the system being characterized in comprising -at least one thickness-measuring detector that is arranged in front of the nip formed by the first and the second roll in relation to the web's direction of motion, -at least one pressure sensor in the first roll for measuring the compression pressure in the nip, -at least one thickness-measuring detector that is arranged behind the nip formed by the first and the second roll in relation to the web's direction of motion, - a means for transmitting and processing the data collected by the above-mentioned devices, and -a means for regulating the thickness of the web on the basis of the data collected and processed.

The thickness of the paper can be measured by any way known per se by those skilled in the art, for example, by a quick-operated laser instrument, a sensor based on IR spectroscopy or any of the methods described above. According to a preferred embodiment of the invention, the thickness is measured by using a non-contact measuring device. When the system is used in the press section of the paper or board machine, the non-contact measuring device can be used, when the dry content of the web is 35-50%.

The thickness-measuring sensors are more preferably placed side by side in the cross machine direction of the paper web, so that quick measurement of the cross direction profile can be carried out throughout the width of the web. In addition, the

measuring device is preferably so small that it can be placed as close to the nip as possible. The thickness gauges located in front of and behind the nip are preferably also identical. Furthermore, the thickness-measuring sensors can be placed in the measuring bar, for example.

Measuring the compression pressure in the roll nip can also be implemented by us- ing any measuring method known per se by those skilled in the art, for example, by piezoelectric sensors or sensors based on magneto restriction or by electromechani- cal films (EMFi). The EMFi films are about 0. 05 mm thick elastic plastic films that have a permanent electric charge. Furthermore, the compression pressure can be measured by any other method described above. The sensors can be located on the roll in any way known per se by those skilled in the art, for example, in rows or in the form of a uniform pattern, unevenly distributed throughout the area of the roll or as a helix pattern. The sensors are preferably located side by side throughout the width of the roll ; the sensors are more preferably located throughout the width of the roll as at least one, more preferably two opposite, even, longitudinal patterns.

When polymer rolls are used, the sensors can be integrated between the layers of coating.

When locating the pressure sensors, it should be taken into account that the com- pression pressure, which is exerted on the roll surface and, consequently, on the pa- per web, should be recorded at each cycle of the roll or at sufficiently frequent in- tervals.

The connection between the pressure sensors and the data processing and control units can be implemented by any cordless data transmission method known per se by those skilled in the art, or by conductors. If the data transmission is arranged through conductors, the rotary parts of the roll can be in sliding contact with the stationary part that receives the signal. The connection between the thickness gauges and the data processing and control units can similarly be implemented by any wireless data transmission method known per se or by conductors. The data processing and control unit collects the data from the first thickness gauge, the pres- sure gauge, and the second thickness gauge, combines and processes the data and uses it to control the process.

According to one embodiment of the invention, a pair of calender rolls in multi roll calendering comprises a polymer roll and a hard roll, whereby the pressure sensors are arranged on the flexible surface layer of the polymer roll or between the layers.

In the set of rolls of the paper machine, the method and the system according to the invention provide accurate and quick control of the paper web's profile. The method can be used to both control the cross direction profile and the longitudinal profile of the web. The method provides particular advantages in paper machines that fore, quently change the grade. The method according to the invention provides accurate management of the process, and the production loss decreases.

A further object of the invention is an on-line method for controlling the calender- ing of the paper or board web in the nip between the pair of calender rolls, the method being characterized in that -the web thickness is measured by one or more thickness sensors placed before the nip and remeasured by one or more thickness sensors placed after the nip, and that -the compression pressure exerted on the web in the nip is measured by one or more pressure sensors arranged in the roll to correlate any change in the thickness of the web and the compression pressure.

The method provides the advantage that the effect of the compression pressure in the nip on the change in the web thickness is accurately known, whereby the effects of a quantitative change in the compression pressure can be predicted. The method gives an opportunity to regulate the web thickness quicker and more reliably than before with the aid of regulated compression pressure in the nip. In addition, con- tinuous data about the web thickness is obtained, which can be used in any other control actions of the process.

The compression pressure of the nip can be regulated in any way known per se by those skilled in the art, for example, in the way disclosed in the publication WO 98/50628.

In the method, the thickness profile of the web is preferably measured by using sev- eral thickness sensors arranged side by side in the cross machine direction of the web. Similarly, the pressure distribution is preferably measured in the cross ma- chine direction of the web by pressure sensors, which are arranged side by side in relation to the web. The pressure distribution is more preferably measured through' out the length of the roll. The measurements enable selective control actions in the cross machine direction of the web, for example, to keep the thickness of the web constant.

The invention is described in detail with the aid of the appended drawings, wherein

Fig. 1 shows a pair of rolls in a system according to the invention, Fig. 2 shows in detail the measurements applied to the web in the system according to the invention, Fig. 3 shows an example of applying the invention to a multi roll calender, and Fig. 4 shows one way of locating the pressure sensor in a polymer roll.

) Fig. 1 shows a nip between the rolls of a pair of rolls in the equipment, which the invention is applied to. The equipment comprises two rolls 1, 2, the roll 1 being a polymer roll with a soft surface and the roll 2 a metal roll with a hard surface. Fur- thermore, the equipment comprises two identical two-piece thickness-measuring members 3, 4, which are located on both sides of the nip 5 formed by the rolls 1 and 2. The thickness-measuring members 3 and 4 comprise several adjacent thickness sensors, which measure the thickness of the paper web 6 before and after the nip 5, respectively (the direction of motion of the paper web 6 is shown by an arrow 7).

Furthermore, the equipment comprises pressure-measuring members 8, 9 located on the polymer roll 1, comprising several adjacent pressure sensors that are located in two opposite rows on the roll 1. The equipment also comprises a wireless arrange- ment for transmitting data from the sensors of the roll 1 to a data processing and control unit 10 (for the sake of clarity, shown by a dashed line in the figure), and conductors 12 and 13 from the thickness-measuring members 3 and 4 to the data processing and control unit 10.

Fig. 2 shows the measurements applied to the paper web in the equipment according to the invention. Data about the pressure exerted on the nip is obtained from pres- sure sensors 30, 31, 32 both in the cross machine direction (curve 14, pressure P) and the longitudinal or the machine direction (curve 15, pressure P) of the web. On the other hand, data about the thickness profile of the paper web is obtained from the thickness-measuring member 4 both in the machine and the cross machine di- rection (curve 16, thickness h of the paper web). These data are combined in the data processing and control unit 10.

Fig. 3 shows an example of applying the invention to a multi roll calender. For clar- ity's sake, the figure has been simplified by omitting some reference numbers and lines that depict the data transmission. The calender comprises three polymer rolls 17, 18, 19 with a soft surface and two hard rolls 20, 21 that alternate with the soft rolls. Pressure-measuring members 8, 9 are arranged in the polymer rolls 17, 18, 19, and the thickness-measuring members 3, 4 are arranged before and after the roll

nips in the way described in Figs. 1 and 2. It is obvious to those skilled in the art that the said measuring members can also be located in some other way than that which is presented in the figure.

The data collected from the pressure sensors and the thickness sensors are directed to a data processing unit 22, from which the combined data is transmitted to a con- trol unit 23. A control signal is transmitted from the control unit 23 to the control system of the calender, which comprises a roll 24 that can be used to alter the pres- sure exerted on the nip, and two heating devices 25, 26 that can be used to alter the surface profile of the hard rolls 20, 21.

Fig. 4 shows one way of locating the pressure sensor in the polymer roll. The pres- sure sensor 8 is arranged between the flexible coating 27 and the intermediate layer 28 of the roll. The roll also comprises a bottom layer 29.